Candling Chicken Eggs To Test Their Viability

Whether you have put your setting of eggs under a broody hen or into an incubator, candling chicken eggs is the way that you visually inspect the development and integrity of each egg using a bright beam of light.

Candles, acetylene bicycle lamps, shafts of sunlight and so on have all been used historically to provide this ray of light which has typically been focused through a hole in a screen or in the side of a box.

These days you can obviously buy excellent and easy to use battery or mains operated devices ( ‘candlers’) for candling eggs, including both hand-held torches and table-top models. Or you can easily make your own.

To start candling eggs, pick up an egg lengthways between your forefinger and thumb. Then hold it directly up to and over the light exit hole in the candler.

In a darkened room and with little or no light escaping from the candler, the egg will hopefully light up like a miniature (but dull)  lamp bulb and you will be able to make out shapes on the inside.

With practice and experimentation at candling chicken eggs, you will soon find out the best way to position and view eggs.

What You See When Candling Chicken Eggs

All eggs vary in shell thickness and colour and both these factors significantly effect the transmission of light through the egg.

Pale coloured eggs are fairly easy to candle but unfortunately dark brown eggs can be very challenging.

Fresh eggs and infertile eggs:

  • These show as clear and translucent when candled.
  • Under a powerful light the outline of the yolk is visible.

Addled eggs (fertile but decomposing):

  • Cloudy throughout.
  • Darker in some places and more or less spotted.
  • Irregular lines can sometimes be seen at the sides.

Fertile eggs:

  • Embryos develop and look like a dark body with radiating spider-like blood vessels.
  • The air cell is visible at the wide end and increases in size with incubation time.

The Schedule For Candling Chicken Eggs

Candling eggs is very advisable when you are selecting your eggs for hatching. This will show up any hairline cracks or double yolks which should be rejected.

Incubation Day 7:

  • Candle eggs and remove any clear eggs
  • If the shells are darker or extra thick, test again on day 10

Incubation Days 14 and 19:

  • Candle eggs and remove any eggs that have not progressed

Be alert for any hint of an unpleasant smell. Fertile eggs which fail to develop may have started to decompose and give off a noxious gas!

These ‘addled’ eggs should be removed as soon as possible and be disposed of carefully. ‘Bangers’ are not a happy prospect!

Final Thoughts

A fast and efficient assessment is the objective when candling chicken eggs. Most light sources also radiate heat … and you don’t want to cook your eggs!

Also candling eggs saturates the developing embryo with an unnaturally high intensity of light. So maybe don’t invade the baby chicks privacy for too long, especially as light can trigger biochemical changes in animals.